Fibers resistant to bending and small diameter
Fibers resistant to bending and small diameter
“Standard single-mode” G.652 fiber has been widely used for decades. The history of the development of standards for optical fibers is based on industry problems and has led up to today. Today we are looking at “bend resistant fibers” and “fine diameter fibers”.
Fibers insensitive to bending attenuation
Bending loss can be a significant problem when installing optical fibers in confined spaces in the access and transmission portions of a network. This is why the ITU developed the G.657 standard, which defines the bending strength of two types of fibers. Class B covers access network fibers that may not comply with the G.652 standard and have low loss when bent to a 7,5 or 5 mm radius (14).
What is a bend-insensitive fiber?
Bend loss occurs when single-mode fibers encounter bends or tight stacks, such as inside cabinets, cable conduits, risers, and bulkheads. One way to limit attenuation is to reduce the mode field diameter to improve optical confinement.
Another approach is to integrate a layer of low refractive index glass into a recessed inner cladding near the core, or into a “trench” within the cladding. Other solutions involve integrating holes or nanostructures of lower wavelength into the fiber core.
The location of these structures is indicated. Choosing the best option depends on the application requirements. Bending loss also increases with wavelength transmission (15).
Small diameter fibers
Reducing the thickness of the fiber (sheath diameter and coating thickness) allows the fiber to be compressed into a smaller volume and bent to a tighter radius without forming the tiny cracks that can cause the fiber to break. fiber. This also allows more fibers to be introduced into the cable. There are two possibilities: reduce the sheath and the protective coating that covers it, or simply reduce the thickness of the protective coating.
How reducing the cladding diameter changes the size of a 10 µm core single-mode fiber.
Standard fibers have an outer diameter of 125µm and a core diameter of approximately 10µm. It is possible to reduce the diameter of the cladding to 80 µm, which reduces the glass volume of the fiber by a factor of 2,4. Reduced-clad, plastic-coated fibers have an outer diameter of approximately 170 µm, compared to 250 µm for a normal-coated fiber.
It is also possible to reduce the thickness of the coating applied to the standard cladding by 125µm, so that the diameter of the coated fiber is only 200µm instead of the usual 250µm.
